The present invention relates generally to cryocoolers and, more particularly, to innovative cryocooler motors having both moving and stationary internal return iron elements or, stated somewhat differently, a xe2x80x9csplitxe2x80x9d return iron element.
Recently, substantial attention has been directed to the field of superconductors and to systems and methods for using such products. Substantial attention also has been directed to systems and methods for providing a cold environment (e.g., 77xc2x0 K. or lower) within which superconductor products such as superconducting filter systems may function.
One device that has been widely used to produce a cold environment within which superconductor devices may function is the stirling cycle refrigeration unit or stirling cycle cryocooler. Such units typically comprise a displacer assembly and a compressor assembly, wherein the two assemblies are in fluid communication and are driven by one or more linear or rotary motors. Conventional displacer assemblies generally have a xe2x80x9ccoldxe2x80x9d end and a xe2x80x9chotxe2x80x9d end, the hot end being in fluid communication with the compressor assembly. Displacer assemblies generally include a displacer having a regenerator mounted therein for displacing a fluid, such as helium, from one end, i.e., the cold end, of the displacer assembly, to the other end, i.e., the hot end, of the displacer assembly. The piston assembly functions to apply additional pressure to the fluid, when the fluid is located substantially within the hot end of the displacer assembly, and to relieve pressure from the fluid, when the fluid is located substantially within the cold end of the displacer assembly. In this fashion, the cold end of the displacer assembly may be maintained, for example, at 77xc2x0 K., while the hot end of the displacer assembly is maintained, for example, at 15xc2x0 K. above ambient temperature.
Now, in situations where it may be desirable to drive the drive the compressor and displacer assemblies using a single linear motor, and where it is desired to have the compressor assembly operate at a fairly high oscillation frequency, such as 60 Hz, several issues arise with regard to the design of the motor assembly. For example, it has been found that, when a conventional linear motor is used to drive the compressor and displacer assemblies of a cryocooler, it is extremely difficult to achieve a 60 Hz operating frequency for the compressor and displacer assemblies. One reason for this is that the diameter of the piston of a given compressor and the amount of displacement achieved by the displacer in a given displacer assembly dictate, to a very large degree, what the maximum amount of moving mass within an associated motor may be. If the moving mass is too great, the motor and associated compressor and displacer assemblies will not function properly.
Thus, it is believed that those skilled in the art would find a linear motor capable of operating at, for example, a 60 Hz resonant operating frequency, when coupled to a piston assembly used in a typical cryocooler to be quite useful.
The present invention is directed to a cryocooler motor having a split return iron element, i.e., a moving internal return iron element and a fixed or stationary internal return iron element. In one innovative aspect, a cryocooler motor in accordance with the present invention is operable at, for example, a 60 Hz resonant operating frequency when used to drive a compressor piston of a cryocooler assembly. Such a motor may comprise an external return iron element, a plurality of coils, a moving internal return iron element, a plurality of magnets affixed to the moving internal return iron element, and a stationary internal return iron element. Moreover, by splitting the internal return iron mass into a stationary element and a moving element it is possible to substantially reduce the amount of moving mass within the motor, and to allow the motor to operate at increased oscillation frequencies. In addition, by reducing the amount of moving iron mass within the motor, it is possible to significantly reduce the wear rate of the parts within the motor.
In another innovative aspect, a cryocooler system in accordance with a preferred form of the present invention has a compact, unitary motor and compressor assembly, thus reducing the size of the overall cryocooler system and enhancing the utility of such a cryocooler system in, for example, tower mount applications. Moreover, in a preferred form, a cryocooler system in accordance with the present invention may comprise a unitary compressor and linear motor assembly, a heat exchanger unit, and a displacer assembly.
The unitary compressor and linear motor assembly preferably comprises a compressor housing that provides structural support for both a compressor assembly and the elements of a linear motor. With regard to the provision of a linear motor, the compressor housing preferably has affixed thereto at least one external return iron element and a plurality of coils, the plurality of coils being fixed between an outer wall of the compressor housing and an inner surface of the external return iron element. The compressor housing also preferably has fixed therein at least one stationary internal return iron element such that a predetermined gap is provided between an exterior surface of the stationary internal return iron element and an inner wall of the housing. The compressor assembly preferably comprises a cylinder and a piston assembly, wherein the piston assembly is slideably disposed within the cylinder and is capable of linear translation along a central axis of the cylinder. The piston assembly preferably has affixed thereto at least one moving internal iron element, which has a length extending into the gap formed between the exterior surface of the stationary internal return iron element and the inner wall of the compressor housing. A plurality of magnets is affixed to the moving internal iron element. The displacer unit preferably includes a displacer that is coupled to a displacer rod. The displacer rod is slideably mounted within a central cavity of the piston assembly such that the displacer rod and piston assembly are capable of independent linear translation along the central axis of the compressor housing. In one particularly preferred embodiment, the displacer rod and piston assembly oscillate at a resonant frequency of approximately 60 Hz, and the motion of the displacer rod leads the motion of the piston assembly by approximately 90xc2x0.
In still another innovative aspect, a unitary linear motor and compressor assembly in accordance with the present invention may include a plurality of gas bearings that reduce and ideally eliminate friction between a piston and cylinder comprising the compressor portion of the unitary assembly. Pressurized gas may be passed, for example, through a check valve into a sealed interior of the piston, thus providing a source of pressurized gas for the gas bearing that does not fluctuate significantly with the pressure of any gas that resides in the compression chamber of the compressor assembly.
Accordingly, it is an object of the present invention to provide an improved cryocooler and an improved compressor and motor assembly for use within a cryocooler.
It is also an object of the present invention to provide a cryocooler motor that is capable of operating, for example, at a resonant frequency of 60 Hz within a cryocooler system, and to provide a cryocooler motor that may have a reduced wear rate.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.